Vulcanized rubber, which is typically in the form of used tires, continues to be an environmental concern because vulcanized rubber consumes landfills all over the world. The demand for new vulcanized rubber articles (e.g., tires) continues to rise, which places burdens on natural resources. Although attempts have been made to reuse vulcanized rubber, these attempts have largely proved to be technologically and economically inefficient. Indeed, the demand for certain recycled rubber products has fallen from about 100,000 tons per day in 1976 to about 200 tons per day in 1995, while the need for new rubber has risen to over 7,000 tons per day for tires alone in the United States.
One attempt to recycle tires includes depolymerization. In one depolymerization process, tires are ground into a crumb having a rather small particle size (e.g., 1/32″). This crumb is then placed in a batch pressure cooker with cracking oils or salts and heated to allow the additives to penetrate the crumb. Once this batch process is completed, steam and volatile organic compound residuals are vented to the atmosphere (or treated with cost prohibitive measures), and the processed rubber is recovered. Besides the air emissions, this process has other shortcomings. The necessity to grind the tire to such a small crumb size consumes unnecessary time and energy. Indeed, the crumb size necessary to separate the fiber and metal from the vulcanized rubber of tires is only about 1/16 of an inch, and it has been estimated that four times the energy is required to further reduce a 1/16″ crumb to 1/32″ particle.
Another depolymerization process employs a continuously flowing hydraulic column. Rubber crumb is suspended in water and is allowed to vertically descend down an annulus of the hydraulic column while the suspension is heated and gravity pressurized. Steam and cracking oils are introduced at the bottom of the GPV to effect the depolymerization, and then the treated rubber is directed out of the GPV though a different annuls where it is cooled and depressurized. Noxious air emissions are reduced by cooling the process stream within the GPV. While this process may handle large volumes of rubber, the resultant product has inherent limitations. Namely, the depolymerized rubber has different characteristics than conventional uncured rubber. While depolymerized rubber can provide rubber compositions with increased adhesive properties and tires having improved life, the viscoelastic properties of the depolymerized rubber is believed to be inferior to virgin rubber.
Attempts have also been made to devulcanize (rather than depolymerize) rubber. Devulcanization severs carbon-sulfur bonds of vulcanized rubber whereas depolymerization severs carbon-carbon bonds. One devulcanization process includes subjecting a dry stream of rubber crumb to heat and mechanical compression using a screw feed. The stream of rubber crumb is directed to a treatment point where the stream is subjected to an oscillating head that produces ultrasonic energy. This treatment is believed to sever the carbon-sulfur bond because the resultant product is soft and behaves like uncured rubber. These processes, however, have severe process limitations inasmuch as the effective zone of devulcanization is very small, and therefore very small amounts of materials can pass through the apparatus. Further, the absence of any practical means to recover the investment in heating and pressurization makes these processes economically impractical.
Thus, there is a continuing need to develop processes and apparatus for the treatment of vulcanized rubber to make reuse and recycling thereof technologically and economically feasible.